Traditionally, hearing devices were powered by primary cells such as zinc-air batteries, directly connected to the circuitry of the hearing device. Since the circuitry of such a hearing device is directly connected to the battery, the state of charge of the battery can be directly measured by the hearing device circuitry by measuring the voltage produced by the battery, and the user can be informed of the state of charge of the battery and be alerted once the voltage, and thus the state of charge, drops below a predefined level, indicating that the battery should be replaced. Replacing the battery frequently is expensive and can be difficult for persons with physical limitations.
More recently, rechargeable batteries have found use in hearing devices. Rechargeable batteries, also known as secondary cells, eliminate the requirement for frequent battery replacement, since they can be recharged, e.g. either by connecting the hearing device to a suitable external power supply, or by wireless transmission of power. This simplifies usage of the hearing device for the user. However, unlike primary cells, secondary cells can generate relatively high voltage levels, e.g. greater than 1.5 V, which can damage or destroy the sensitive circuitry of the hearing device. In consequence, a battery power management system is often used to control charging of the battery and also to ensure constant voltage levels of the power supplied to the circuitry such that this voltage level does not exceed a threshold which may damage this circuitry. However, since the battery power management system ensures a constant voltage to the circuitry of the hearing device, this can no longer measure the state of charge of the battery by directly monitoring its output voltage, since it is no longer directly connected thereto. It is thus possible that the user may be surprised by sudden powering down of the hearing device without warning due to the battery being exhausted.